1. Field of the Invention
The present invention relates to an isolated switching power supply apparatus where required isolation between the primary side and the secondary side is achieved by using digital control circuits, such as digital signal processors (DSPs), as control circuits.
2. Description of the Related Art
In switching power supplies using a commercial power supply apparatus as an input, it is mandatory to ensure isolation between the primary side and the secondary side of a transformer for safety reasons. To control an output voltage or output current on the secondary side, a feedback circuit detects the output voltage or output current and feeds it back to the primary side so as to control a switching circuit on the primary side. The feedback circuit itself is also required to ensure isolation between its primary and secondary sides. See, for example, International Patent Application Publication 2009/011374.
In recent years, attention has been paid to “digital control power supplies” using a digital control circuit, such as a microprocessor or DSP, as a control circuit for controlling a primary-side switch. The digital control circuit for controlling a primary-side switching circuit is generally disposed on the primary side. In the case where the input voltage is high, it is necessary to use a digital control circuit composed of a high-voltage semiconductor device, or to provide the transformer with another winding to generate a voltage for driving the digital control circuit. On the other hand, there is a desire to dispose a digital control circuit on the secondary side (load side) to utilize a merit thereof, the communication function. Specifically, in order to transmit or receive various types of information to or from a load circuit or external device, it is preferable that a digital control circuit be disposed on the secondary side.
FIG. 1 shows the basic configuration of an isolated DC-DC converter disclosed in International Patent Application Publication 2009/011374.
In FIG. 1, a transformer T1 includes a primary winding N1 and secondary windings N21 and N22. A switching circuit SW composed of four bridge-connected switching elements QA, QB, QC, and QD and an inductor L1 are connected to the primary winding N1. A filter circuit composed of a common-mode choking coil CH and bypass capacitors C1 to C6 and a current transformer CT are provided between an input power supply 1 and the switching circuit. A resistor R3 and a rectifier diode D3 are connected to the secondary side of the current transformer CT so that current flowing into the primary side thereof is outputted to the resistor and the rectifier diode as a voltage signal.
A drive circuit 11 is connected to the four switching elements QA to QD of the switching circuit SW.
A rectifying smoothing circuit composed of rectifier diodes D1 and D2, an inductor L2, and a capacitor C7 is connected to the secondary windings N21 and N22 of transformer T1. The rectifying smoothing circuit outputs an output voltage to output terminals T21 and T22. A load circuit 2 is connected between the output terminals T21 to T22. An output voltage detection circuit composed of resistors R1 and R2 are provided between the output terminals T21 to T22.
A digital control circuit 10 is composed of a DSP and is provided on the secondary side. The digital control circuit 10 outputs a control pulse signal for the switching circuit SW to a pulse transformer T2. The drive circuit 11 receives the above-mentioned control pulse signal via the pulse transformer T2 so as to drive the switching elements QA to QD of the switching circuit SW.
On the basis of the rising timing and the falling timing of the control pulse signal in the pulse transformer T2, the drive circuit 11 phase-controls the drive pulse signals of the switching elements QA to QD so as to turn on or off a set of the switching elements QA and QD and a set of the switching elements QB and QC alternately.
As shown in FIG. 1, in an attempt to control the primary-side switching circuit by disposing the digital control circuit on the secondary side, use of the pulse transformer T2 can reduce the size of the transformer. However, since a pulse transformer generally has a small inductance value, it can transmit only a signal corresponding to the edge of the pulse waveform. That is, the pulse transformer transmits only a timing signal. This creates a need to provide, on the primary side, a drive circuit that receives the timing signal to actually generate a signal for driving the switching circuit. Omission of this drive circuit creates a need to transmit a pulse-shaped drive signal itself from the secondary side to the primary side. This requires a transformer having a large inductance value (drive transformer), preventing downsizing of the transformer. Moreover, use of an isolating element, such as a photocoupler, disadvantageously causes the deterioration of responsiveness or degradation with time.
In the case where a switching power supply apparatus controlled by a digital control circuit, such as a DSP, is a large-capacity power supply apparatus using a commercial power supply apparatus as an input, it is virtually essential to incorporate a PFC (power factor correction) converter into the switching power supply apparatus so as to satisfy the harmonic current regulation. Further, in a DC-DC converter including a PFC converter, control of both the PFC converter and the DC-DC converter by a single digital control circuit disadvantageously requires the digital control circuit to have a high high-speed processing capability.